Microwave frequency combs (MFC) have been generated using step-recovery diodes (SRD) and non-linear transmission lines (NLTL), as well as using photodetectors (PD) and other methods requiring lasers. Both SRD and NLTL are used in low-noise applications, and have measured linewidths of greater than or equal to 1 kHz. Step-Recovery Diodes have been used to generate as many as 100 harmonics at frequencies up to 50 GHz, and NLTL shows promise for applications at higher frequencies. Semiconductor photodetectors having bandwidths as high as 100 GHz are used with microwave spectrum analyzers to characterize ultrafast lasers, and photodetectors are also used in low-noise applications, frequently mounted on an antenna where they are referred to as “photoconductive antennas”. Other methods for generating microwave frequency combs that require lasers include optoelectronic feedback by injecting the optical pulses to a slave laser, and coupling the detected microwave output to a microwave synthesizer for negative feedback. Typically the line widths are also greater than or equal to 1 kHz for these laser methods.
The fractional linewidth of the peak in a signal, defined as the full-width at half height divided by the center frequency for the oscillator generating the signal, is equal to the reciprocal of the quality factor or Q of the oscillator. The highest reported quality factors are approximately 109 at a single frequency between 10 GHz and 20 GHz generated using cryogenic quartz bulk acoustic wave resonators or cryogenic sapphire oscillators.
Microwave energy has been coupled into and/or out of tunneling junctions in scanning tunneling microscopes (STM) using separate coaxial cables connected to the tip and sample electrodes of the STM, or by using a coil in close proximity to the tunneling junction. These methods provide adequate coupling at frequencies up to several GHz, but the coupling region is significantly larger than the tunneling junction which increases the noise.
Scanning capacitance microscopy (SCM) has been used for nanoscale dopant profiling in semiconductors, where fringing capacitance and stray capacitance constitute the bulk of the measured capacitance, and capacitance of the depletion region in the semiconductor sample to be measured, represents about 1 part per million of the total capacitance. Thus, small changes in the total capacitance must be determined using a resonant circuit. Tip electrodes having radii of curvature less than 10 nm are difficult to fabricate; therefore, 10 nm is presently a lower limit for resolution in measurements that are performed using SCM.